Everything in a spiral galaxy rotates around the center of the galaxy at the same rate. It’s as if every disk-like galaxy were a giant phonograph record, everything rotating in lock-step. It’s one of the great discoveries in astronomy, made by Vera Rubin. She probably should’ve gotten the Nobel Prize for it, but never did. She made it by studying stellar red shifts (fortunately, they don’t have to watch the galaxy rotating for tens of millions of years before getting a fix on the rotation rate!)
“It’s as if every disk-like galaxy were a giant phonograph record, everything rotating in lock-step.”
Not so.